ML20073B397

Submits Comments on Seismic Confirmatory Program,Vc Summer Station Unit 1, in Preparation for Meeting W/Licensee.Rept Fails to Demonstrate Applicability of Explosion Tests to Earthquakes

ML20073B397
Person / Time
Site:	Summer
Issue date:	04/08/1983
From:	Joyner W INTERIOR, DEPT. OF, GEOLOGICAL SURVEY
To:	King J NRC
References
NUDOCS 8304120326
Download: ML20073B397 (3)

Text

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y;[.% , LaiA$n 2c.bQ &ho- O?d 5 United States Department of the Interior j

$ GE01DGICAL SURVEY 3..

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i 0FFICE OF EARTHQUAKES, VOLCAN0ES, AND ENGINEERING 4

Branch of Engineering Seismology and Geology

'345 Middlefield Road, MS 77 Menlo Park, CA ' 94025 -

April 8,1983 4

i Dr. Jerry L. King i

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U. S. Nuclear Regulatory Commission Mail Stop P-514 i Washington, D. C. 20555

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Dear Jerry:

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{ In preparation for our scheduled meeting with the Licensee later this month, I am setting down my preliminary comments on the report " Seismic j Confirmatory Program, Virgil C. Summer Nuclear Station Unit 1", including the

) Addendum to Appendix B, dated March 1983.

1 i Noel Bycroft will comment on the theory used in interpreting the pad

tests, and I will leave that subject largely to him. I would like to suggest,-

however, that since the modal approach,.which was used to get the final 4

answer, is not completely rigorous, it would' be of great interest to see a j comparison between the observed displacement. amplitude and phase from the low-i force-level test (e.g., Figure 4 of Appendix A) and the predictions of the

] model obtained by the modal analysis. That should be a relatively simple

j. thing to do.

t j With respect to the explosion tests as a means of detennining foundation i effects, I don't believe the submissions demonstrate the-applicability of the j explosion tests to earthquakes. They show that the explosion seismograms are-1 dominated by what they call S-wave and higher mode surface waves. They claim

! to show that the spectral ratios are similar whether computed for a P-wave j window, an S-wave window (including the surface waves), or for the whole.. '

! record. They show that fundamental mode surface waves do not contribute to l

! the spectral ratios. All of this, however, does not demonstrate that the l relative response of foundation and free-field sites is the same for .

earthquakes as for explosions. If we accept the description of the dominant '

l portion of the seismograms as a combination of S-waves and higher mode surface r waves, we have to presume ~ that the relative excitation of the various i components of this combination is sensitive to the depth of the source, and.

l further, that the relative response of foundation and free-field sites is @o (

i. sensitive to the relative excitation of- the components. Looked at from a

. different point of view, it may be more realistic to consider the dominant j portion of the seismogram to be a combination of scattered S-waves, scattered

not only in the vicinity of the recording site, but also in the vicinity of the source and in.the vicinity of any surface reflection points between the 8304120326 830400 PDR ADOCK 05000395 p PDR a

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J source .and the recording site. From this point.of view, one.would expect the  ;

i character of the motion, and in particular, the relative response at foundation and free-field sites to be ' sensitive to the depth of.the source.

Either way, the source depth may be a significant variable. The average depth of the explosions must be significantly less than the average depth of the earthquakes, and all of the explosion depths may in fact be less than all of the earthquake depths. Two shots were at depths comparable to the computed hypocentral depth of the October 1979 earthquake, but no data from those. shots l

{ is shown and, as I interpret Table V.C.1, no data from those fshots was used in i

computing horizontal spectral ratios. In any case, the uncertainty of the computed hypocentral depth forces us to acknowledge that the real depth could be substantially greater than that for all of the explosions. This leaves me j with substantial doubt concerning the applicability of the. explosion tests for determining relative response in an earthquake. (My conern on this point is increased by comparing Figure IV.C.1 with Figures IV.C.3 and ~IV.C.6. The

comparison is difficult to make because of the scale change, but the records of the shots seem to have higher coda amplitudes at free-field sites than the 4

record of the earthquake, and the particle-motion diagrams show a different pattern for the earthquake compared to the explosions.)

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I am also concerned about the use of a zero-phase-shift filter to represent the transfer function between foundation and free-field sites. The effect of this is difficult to assess and could be quite significant.

Relative phase controls the extent to which an increase in modulus represents 1

an increase in peak amplitude and the extent to which it represents an increase in duration. The effect of an increase in amplitude- on damped i

response spectra may be very different from the effect of an increase in duration, depending on the character of the signal. To help us assess this .

aspect of their work, I think the Licensee should be asked to show comparisons of typical observed free-field records with synthetic free-field records formed by filtering Auxiliary Building records with the corresponding zero-phase-shift filter.

i Another major problem in the interpretation of the explosi$n tests is the treatment of variablility. The consultants use the mean spectral ratio. I question this choice in spite of the ingenious argument offered by Robin

• McGuire. As you remember, he argued that, if the mean ratio had turned out to-be 1.0, NRC would not have required that the spectrum be raised; so no extra margin over the mean ought to be required in the case of a mean less than 1.0. I disagree. I think that if the mean had (surprisingly) turned out to be 1.0, prudence would have required giving serious consideration to raising the spectrum. From the standpoint of safety, the question is not whether the spectrum is exceeded on the average, but, rather, whether it is' exceeded anywhere. If the Envelope Spectrum embodied a -large degree of conservatism, it might be argued that the use of the mean spectral ratio.was justified, but the envelope of all events recorded in approximately.a five-year period cannot be considered conservative without some debatable assumption about the rate at which the earthquake sequence is dying off. .

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I cannot find a clear explanation of how the " band pass spectral ratios" for the various windows were calculated. I may just have missed it, but, if not, the consultants should supply it. In comparing the -band pass ratios shown in Figures VI.C.55 and VI.C.58 with the ratios in Figures VI.C.20 and VI.C.24, it does not seem to me that the comparison is very good, particularly in the vicinity of 20 Hz. Similarly, I don't think the agreement is very good between the ratios shown in Figures VI.C.47 and VI.C.48. The consultants .

apparently consider disagreements by a factor of two as matters of trivial importance. That being the case, they will presumably not object to raising their final answer by a factor of two.

The problems discussed above are the most important issues I see at this time. In going through the report I noted some minor problems. At the bottom of page 1, it is stated that the results of the pad tests comfirm testimony at the 1982 ASLB hearings. I do not believe that is the case. You have access to the transcripts and could check. If the statement is not correct, perhaps it should be deleted; otherwise, the record gives a false impression of consistency. I don't understand the last paragraph on page 8. I expect you do, but, if not, it should be clarified. It is not clear to me that the reasons they give on page 9 (paragraph 1) are adequate to show that the Auxiliary Building motions are representative of motions in the Intermediate Building; an engineer should look at this. On page 10, paragraph 2, I don't understand why motions recorded in the sump would overestimate motions expected on the main floor of the Diesel Generator Building. Either Figures IV.A.7 and IV.A.8 are mislabeled or Figure IV.C.3 is mislabeled. The labeling of Figure V.C.1 is inconsistent. Does it represent data recorded at site F3 or site FR? In Table C.V.1 the term " Code Revision f" should be defined. On page 43 of Appendix B, line 13 up from the bottom, a reference is made to Figure VI.C.7 when Figure VI.C.6. is obviously intended. The discussion in the first paragraph of page 43 of the differences between spectral ratios computed for different time windows is entirely in terms of the vertical component. It would have been more relevant if it have been done for the horizontal components.

If you want to discuss any of these problems before the meting, give me a cal l .

Best regards, William B. Joyner Geophysicist Copy to:

Ted Algermissen 3